1. Field of the Invention
The present invention relates to a method for manufacturing a toner designed for use in development of an electrostatic charge image or the like during the course of image formation effected by means of electrophotography or otherwise.
2. Description of the Related Art
In an electrophotographic image forming apparatus for forming images by electrophotographic method, image formation is accomplished in the following manner, for example. After an electrostatic charge image is formed on a surface of an electrophotographic photoreceptor (hereafter also referred to simply as “the photoreceptor”) by various apparatuses, the electrostatic charge image is developed into a toner image by using a supplied toner. Lastly, the toner image is transferred onto a transference material such as a paper sheet and is then fixed into place. The toner used to develop the electrostatic charge image (here after referred to as “the electrostatic charge image developing toner”) is composed of a binder resin having dispersed therein additives such as a colorant and a charge controlling agent. The toner is electrically charged by friction and is then carried on a developing roller or the like means whereby to supply the toner to the surface of the photoreceptor.
In recent years, as research and development have been carried out to design toners having increasingly smaller particle size with the objective of attaining upgraded image quality, such a toner as has a small volumetric average particle diameter ranging, for example, approximately from 3 μm to 8 μm has been coming into wider and wider use. For toner production, a so-called pulverization method has been widely used; that is, a method of obtaining a toner by kneading a binder resin, a colorant, and other additive as required, and then dry-pulverizing the resultant resin kneaded product. In the case of adopting the pulverization method, however, the smaller is the particle diameter of the toner obtained, the more likely it is that the particles will be irregular-shaped. This gives rise to a problem of a significant deterioration in powder fluidity. Such a toner as has poor powder fluidity cannot be supplied to the surface of the photoreceptor with stability in a development process, in consequence whereof there results degradation in image quality.
Moreover, the toner obtained by the pulverization method is liable to suffer from uneven charging capability because of its relatively wide range of particle size distribution. If image formation is carried out by using such a toner as has uneven charging capability, at the time of transferring a toner image onto a transference material, part of the toner cannot be transferred onto the transference material properly due to lack of charge amounts, thus causing an undesirable decrease in image density or the like problem. Occurrence of uneven charging capability in the toner obtained by the pulverization method cannot be prevented without the necessity of carrying out classification after granulation is completed through a pulverization process to narrow the particle size distribution range. However, the classification leads to low toner yield and thus gives rise to another problem of high cost of manufacturing.
As described hereinabove, the pulverization method presents various problems. Therefore, as an alternative, the adoption of a wet method has been examined for toner production. For example, the wet methods include:
(i) a suspension polymerization method for obtaining a toner by polymerizing, in the presence of a colorant, monomer of a binder resin dispersed in water by using a suspension stabilizer, and encapsulating the colorant in binder resin particles to be formed (refer to Japanese Unexamined Patent Publication JP-A 8-305084 (1996) (pages 4 and 5, FIGS. 1 and 2) and Japanese Examined Patent Publication JP-B2 3466872(pages 3 to 5, FIGS. 1 and 2), for example);(ii) an emulsification polymerization-based agglomeration method for obtaining a toner by mixing a water dispersion of resin particles obtained through emulsification polymerization of binder resin monomer and a water dispersion of a colorant or the like to form agglomerated particles of the resin and the colorant, and melting the agglomerated particles with application of heat;(iii) a phase inversion emulsification method for obtaining a toner by dissolving or dispersing water-dispersible resin and a colorant in an organic solvent, adding thereto water and a neutralization agent for neutralizing a dissociation group of the water-dispersible resin with stirring to form resin solution droplets having enclosed therein the colorant and the like, and subjecting the resin solution droplets to phase inversion emulsification;(iv) a dissolution suspension method for obtaining a toner by dissolving or dispersing a toner material containing a binder resin and a colorant in an organic solvent in which the binder resin is soluble, mixing the obtained solution or dispersion with a water dispersion of an inorganic dispersant, for example a less water-soluble alkaline earth metal salt such as calcium phosphate or calcium carbonate to achieve granulation, and removing the organic solvent; and(v) an emulsification dispersion method for obtaining a toner by dissolving or dispersing a binder resin, a colorant, and the like in a water-insoluble organic solvent in which the binder resin is soluble, emulsifying and dispersing the obtained solution or dispersion in an aqueous dispersion, and removing the organic solvent.
However, the above stated methods (i) through (v) present the following problems. For example, according to the polymerization method such as the suspension polymerization method (i) and the emulsification polymerization-based agglomeration method (ii), a polymerization reaction takes place in water. Therefore, a resin material which is usable as a binder resin is limited to vinyl polymer that can be produced by radical polymerization. In consideration of toner fixation property and toner transparency which is required to form a color toner, it is desirable to use polyester resin rather than vinyl polymer as the binder resin. That is, it is preferable that the binder resin is selected properly in accordance with desired characteristics to be fulfilled by a toner to be produced. Accordingly, a toner manufacturing method is sought after that does not necessarily have to use vinyl polymer but can use resin materials of various type.
Another problem associated with the polymerization method is occurrence of uneven charging capability in toner particles resulting from residual binder resin monomer, polymerization initiator, and suspension stabilizer, and so forth remaining within the toner particles. Although occurrence of uneven charging capability cannot be prevented without the necessity of removing such residues, it is extremely difficult to remove the monomer, polymerization initiator, and suspension stabilizer, and so forth incorporated inside the toner particles. Moreover, according to the emulsification polymerization-based agglomeration method (ii), since a toner is produced by melting the agglomerated particles of the binder resin, the colorant, and so forth with application of heat, there arises a problem that toner particles having uniform composition cannot be formed with stability.
Further, according to the phase inversion emulsification method (iii), the dissolution suspension method (iv), and the emulsification dispersion method (v), since an organic solvent is used to dissolve or disperse the binder resin, a solvent collecting apparatus is required in view of an attitude toward environmental issues. This necessitates a large-scale manufacturing facility. Another problem associated with the methods (iii) through (v) is that a resin material which is usable as the binder resin is limited to a water-dispersible resin having a dissociation group or an organic solvent-soluble resin.
As a technique to solve these problems, a so-called fusion emulsification method has been proposed to date (refer to Japanese Unexamined Patent Publication JP-A 2005-165039 (pages 4, 8, and 9), for example). This is a method of obtaining a toner by melting and kneading constituents for toner such as a binder resin and a colorant, mixing the obtained resin kneaded product with an aqueous medium containing a dispersant, and stirring the resultant admixture while applying heat to the aqueous medium contained in the admixture to disperse and granulate the resin kneaded product. According to the fusion emulsification method, it is possible to use resin materials of various type as the binder resin, and thereby allow easy production of a toner having desired characteristics.
As described hereinabove, according to the fusion emulsification method disclosed in JP-A 2005-165039 for example, it is possible to produce a toner having desired characteristics by using resin materials of various type as a binder resin. However, from the standpoint of obtaining a toner having desired characteristics more reliably, further improvement is hoped for in the technique disclosed in JP-A 2005-165039.
According to the fusion emulsification method, heat is applied to the aqueous medium contained in the admixture of the resin kneaded product and the aqueous medium to soften the resin kneaded product. After that, the softened resin kneaded product is pulverized and dispersed by an stirring apparatus, and is thereby granulated. At this time, depending upon a temperature at which the aqueous medium is heated, the melt viscosity of the resin kneaded product may become so low that various components contained in the resin kneaded product, such as a colorant, a release agent, and a charge controlling agent are liable to agglomerate. This could lead to low dispersibility. Moreover, the components dispersed in the resin kneaded product such as the colorant may be separated from the resin kneaded product, which could result in deviation of the composition of a toner to be obtained from the intended composition. Variation in the dispersibility or composition of the components contained in the resin kneaded product gives rise to a problem that the desired characteristics cannot be attained. Accordingly, from the stand point of obtaining a toner having the desired characteristics, it is preferable to adjust the temperature at which the aqueous medium is heated to be as low as possible. However, the lower is the heating temperature for the aqueous medium, the more likely it is that the resin kneaded product will not be melted easily. This increases the possibility of a failure of granulation. Furthermore, even if granulation can be achieved somehow or other, it is difficult to obtain a volumetric average particle diameter ranging from approximately 3 μm to 8 μm, which is suitable for an electrostatic charge image developing toner, and also it is inevitable that the particle size distribution becomes broad.
JP-A 8-305084 and JP-B2 3466872 described previously made the following proposal to attain improved granulation capability. That is, a polymeric monomer constituent containing a polymeric monomer, a colorant, and a polymerization initiator is granulated in an aqueous disperse medium under a shear force, for example, exerted by a rotor and a screen surrounding it. After that, the resultant particles are polymerized by means of suspension polymerization. However, the techniques disclosed in JP-A 8-305084 and JP-B2 3466872 relate only to the suspension polymerization method, and pays no regard to the fusion emulsification method. Accordingly, the techniques disclosed in JP-A 8-305084 and JP-B2 3466872 cannot be applied as they are to the fusion emulsification method.